Abused substances produce long-term brain changes including neurotoxicity, through mechanisms that are largely unknown. The candidate neuromodulator nitric oxide (NO) can mediate aspects of NMDA- induced neurotoxicity in vivo and in primary cultured neurons. In this FY, we have extended observations made during the previous fiscal year that show that in situations of excess glutamate stimulation, NO mediates cell death in culture and therefore may mediate the cellular destruction observed following stroke, brain injury, neurodegenerative diseases, and severe drug abuse. Mature primary culture neurons were exposed to various drug solutions and the resultant cell death was assayed. NMDA neurotoxicity was found in rat primary cell cultures of cortex. Modulating the activity of the NO- synthetic enzyme nitric oxide synthetase (NOS) by inhibiting its cofactor calmodulin, by changing its phosphorylation state or by inhibiting the shuttling of electrons through its flavoproteins modulates NMDA neurotoxicity: as NOS activity is decreased so is neurotoxicity. The toxicity of the HIV-coat protein, gp120, can also be attenuated with inhibitors of NOS.